Resistive card high frequency attenuators having capacitive compensation



Nov. 24, 1970 s, ADAM ETAL RESISTIVE CARD HIGH FREQUENCY ATTENUA'IORS HAVING CAPACITIVE COMPENSATION Filed Oct. 24, 1966 INVENTORS FTV mp MN FN I 1 u J i ii: I 1

STEPHEN F'. ADAM RICHARD W. ANDERSON -Q. gmrixk ATTORNEY United States Patent RESISTIVE CARD HIGH FREQUENCY ATTENUA- TORS HAVING CAPACITIVE COMPENSATION Stephen F. Adam and Richard W. Anderson, Los Altos,

Calif., assignors to Hewlett-Packard Company, Palo Alto, Calif., a corporation of California Filed Oct. 24, 1966, Ser. No. 588,945 Int. Cl. H01p 1/22 US. Cl. 333-81 3 Claims ABSTRACT OF THE DISCLOSURE The attenuation characteristic of a resistive-card coaxial attenuator is rendered substantially independent of frequency over a selected range of operating frequencies by introducing an isolated conductive element adjacent the resistive card to provide distributed capacitive compensation about a region of the resistive card that corresponds to the area of the conductive element.

This invention relates to signal attenuators and has as its principal object the provision of frequency compensation for a fixed resistive film attenuator card. It is another object of the present invention to provide capacitive compensation for the resistive element of a coaxial line attenuator.

This is accomplished in accordance with the illustrated embodiment of the present invention by providing a conductive element in the region of the electromagnetic field about the resistive film element of a coaxial line attenuator card for bypassing a portion of the resistive element at high frequencies.

Other and incidental objects of the present invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIG. 1 is a sectional view of a coaxial line, fixed value resistive card attenuator showing the location of the frequency-compensating capacitor element;

FIG. 2 is a sectional view of the fixed resistive film attenuator element; and

FIG. 3 is a graph showing the improved frequency response of the apparatus of FIG. 1 using the capacitive compensation according to the present invention.

Referring now to FIG. 1 there is shown a coaxial line attenuator including a conductive body 9 fitted at opposite ends with male and female standard coaxial line connectors 11, 13. The body 9 and connectors 11 and 13 include a central bore 15 within which the two sections 16, 17 of the center conductor are coaxially disposed. The body 9 also includes a diametrical slot 19 along the central bore for supporting the resistive film attenuator element in conductive contact with the body 9. This attenuator element 21 as shown in FIGS. 1 and 2, includes an insulating card or substrate 23 which supports a rectangular region of resistive film 25. Electrodes 27, 29 along the edges of the card 23 contact the longitudinal edges of the resistive film 25 to facilitate making good low ohmic contact between the edges of the resistive film 25 and the conductive body 9. The center conductor 16, 17 is interrupted at the resistive film 25 and the sections are conductively attached to electrodes 31, 33 which contact only a restricted area at the ends of the resistive film 25. Attenuator elements of this type are described in US. Pat. 3,227,975 issued on Jan. 4, 1966 to W. R. Hewlett and W. B. Wholey. The frequency response of this type attenuator element typically follows the characteristic curve 35, as shown in the graph of FIG. 3, and includes 3,543,197 Patented Nov. 24, 1970 ice a roll-off with frequency (i.e. a departure from frequencyindependent attenuator) at a frequency of the order of 10 gHz.

The frequency response of the attenuator element 21 is improved according to the illustrated embodiment of the present invention by introducing a conductive element 37 in the region of electromagnetic fields surrounding the resistive film 25. This element 37 which is attached to the card 23 on the under side thereof is insulated from the resistive film 25 by the insulating card 23 and thus serves as an electrode of a capacitor. Of course, the conductive element 37 may also be placed on top of the resistive film 25 provided an intermediate layer of insulation is disposed between the film 25 and the element 37. The capacitive compensation thus provided has a theoretical frequency response as shown by curve 39 in the graph of FIG. 3. The compensating response combined with the normal roll-off response of the attenuator provides the resultant response shown by the curve 41 which is frequency independent at a selected attenuation value 43 to frequencies of the order of 20 gHz. It is believed that this compensation is due to the distributed capacity that is provided between the ends 16, 17 of the center conductor and the conductive element 37 which becomes small in impedance value as the frequency of the applied signal increases. This compensates for the increase in in ductive reactance with signal frequency in the signal path through the film 25 between the sections 16, 17 of the center conductor. Also the distributed capacity between unit areas of the film 25 and the conductive element 37 tends to bypass the resistive path between the sections 16, 17 of center conductor at higher frequencies effectively to shorten the resistive and inductive path length between the sections of the center conductor. As a result, the distributed capacitance of element 37 and the inductance of the resistive film 25 due to current flow between conductor sections 16, 17 resonate at a frequency well above the frequency band of interest. Thus, the area of the conductive element 37, the dielectric constant and thickness of the insulating layer between the resistive film 25 and the element 37 may be chosen empirically to establish resonance at a selected frequency and thereby establish the desired amount of compensation (curve 39) necessary to flatten the response curve 41 as shown in FIG. 3.

We claim:

1. Attenuator apparatus comprising:

coaxial conductor means having a first outer conductive grounded member and a first inner conductive current member coaxially spaced and insulated from said first outer member;

an insulating member having at least one surface;

a layer of resistive material on said surface, the periphery of said layer on the surface having two opposed and spaced sides and two opposed and spaced ends;

means conductively connecting the resistive material substantially along the length of at least one of said sides to the first outer conductive member;

means conductively connecting the first inner conductive member to the resistive material at a restricted area adjacent one of said ends and spaced from said sides; and

a conductive element insulated from said resistive material and from said first inner and outer conductive members disposed adjacent the resistive material intermidate the opposed ends and opposed sides thereof for providing high frequency compensation that extends the range of operating frequencies over which signal attention is substantially constant.

2. Attenuator apparatus as in claim 1 comprising:

second coaxial conductor means having an outer conductive grounded member electrically connected to said first outer conductive grounded member and having a second inner conductive current member c0- axially spaced and insulated from said second outer conductive member;

means conductively connecting said second inner conductive member to said resistive material adjacent a second restricted area spaced from said first restricted area and spaced from said sides; and

said conductive element is insulated from said second inner and outer conductive members.

3. Attenuator apparatus as in claim 1 wherein said insulating member has another surface spaced away from and substantially plane-parallel with said one surface; and said conductive element is disposed on said other surface of said insulating member located intermediate the spaced sides and spaced ends.

References Cited UNITED STATES PATENTS 7/1958 Vallese 333-81 11/1960 Tanenbaum et a1 333-81 10/1961 Weinschel et al. 333-81 9/1963 Norman 333-81 11/1964 Weinschel 333-81 11/ 1965 Isaacson 333-84 1/1966 Hewlett et a1. 333-81 FOREIGN PATENTS 5/ 1958 Australia.

US. Cl. X.R. 

